I can set my 30 year old expensive Polish Vernier caliper to 25.4mm or 1.0"......[so easy to read] and the repeatability of my el-cheapo digital caliper to display the same dimension over the blades of the vernier is faultless

it is important to understand the latter are only an expensive G Clamp with a non adjustable TV box in the corner, however checking the accuracy and repeatability of a digital caliper [through comparison] will provide confidence in their use especially for high rate of finished component back checking

Modern component manufacturing Drawings will normally provide a dimension then a +/- tolerance or any requirement beyond that......many older Drawings offer whole number...eg., a 1" bore.....so is the bore machined & reamed with a 1" parallel reamer?......do you confirm the bore dimension with telescopic gauges...then measure these with external micrometers?

If the piston was nominated as 1", is it machined to provide 0.001" per inch clearance?

How many home workshops have test pieces to enable the calibration of these micrometers @ 20 degrees C?, in Industry ...the person conducting the checking and calibration of all measuring devices must be qualified in the work

Another factor in the performance of digital callipers or any digital scale for that matter is the batteries. For any given size and voltage standard, there are two types of battery chemistry available, alkaline and silver oxide. Both are fine in the suitable application, but the silver oxide has a much more stable voltage characteristic, so is preferred for measuring type applications. I know that the first sign of low batteries on my Measuremax instruments is poor repeatability of the readings, and the silver oxide work well for much longer than alkaline batteries.

I don't know if the top quality instruments have circuitry that compensates better or are just normally supplied with the silver oxide type anyway. I am sure that better batteries will not bring a cheap device up to the quality of the better instruments, but they reduce some of the frustration in the range where they are adequate. It may be worth trying a better battery before consigning them to the bin if you have already bought them. Of course better to buy the best you can afford rather than the cheapest available. But for final cuts and best accuracy, I am another who resorts to the traditional micrometer, a beginner lesson learned the hard way.

(The alkaline ones are fine for less critical applications, such as LED lights and music devices, but the voltage drops off with use more than the silver oxide, so not good for accurate measuring instruments of any kind.)